The present invention relates to an elastically deformable press cover of a press for dewatering a web material, in particular a press cover of a dewatering press for a paper making machine, or the like. The press cover has ridges on its outside, which faces the web, and between the ridges where there remain grooves which are open to the outside.
Press covers of this kind for dewatering presses are used, for example, in the press section of a paper making machine, for pressing water out of a wet paper web and conducting the water away. In such an arrangement, the paper web is passed through a press zone, together with a dewatering felt, or is alternatively sandwiched between two dewatering felts. The press zone can be formed from two rotatable press rolls which meet at a nip. At least one of the press rolls has an elastically deformable press cover which is provided with grooves on its exterior. The press zone can alternatively be formed by a single rotatable press roll which cooperates with and presses against a pressure shoe. The pressure shoe presses a press cover against the press roll so that the press roll co-revolves with the press cover. This arrangement forms an extended press zone, which is extended in the direction of web running. In either case, water is squeezed out of the paper web in the press zone and is released directly to at least one dewatering felt that is moving through the press zone along with the web. Part of the water from the web passes through the pores of the felt and into the above mentioned grooves in the press cover.
Grooves of this kind are provided in press rolls particularly for relatively high paper web running speeds, i.e. relatively high production speeds. Grooved press rolls are known in which the grooves are worked into a metallic press roll cover. In these press rolls, the cross-sectional shapes of the ridges and the grooves remain unchanged in the press zone. However, the specific compression in the press zone is relatively high, creating a danger that the paper web will be crushed. In contrast, when an elastic press cover is used, the specific compression remains at an acceptable magnitude, even at a relatively high linear force. However, the ridges between the grooves are spherically deformed or rounded in the press zone, and the grooves are at the same time narrowed due to the deformation of the ridges. This has the disadvantage that the water space capacity of the grooves is reduced and the flow off of water through the grooves is hindered.
The present invention is concerned with an elastic press cover. A press cover of this kind can be applied as a fixed covering on a rotatable roll element. See U.S. Pat. No. 4,353,296 and German Pat. No. 2,814,682. Alternatively, the press cover can be designed as an endless, flexible belt or as a tube. The outside of the belt or tube is pressed against a mating press roll by means of a pressure shoe or by means of a rotating roll element behind or inside the press cover. See German Offenlegungsschrift No. 3,501,635, which is equivalent to U.S. Pat. No. 4,625,376, and see U.S. Pat. Nos. 4,552,620 and 4,238,287.
With known elastic press covers, measures have been proposed for avoiding the narrowing of the grooves in the press zone. FIG. 2a of U.S. Pat. No. 4,353,296 suggests that the deformation of the ridges be avoided or at least reduced by using materials for the cover having anisotropic properties. Thus, the modulus of elasticity measured transverse to the direction of running is said to be greater than the modulus of elasticity measured in the direction of running. However, it is doubtful whether this proposal avoids ridge deformation and whether it can be implemented with a reasonable input of manufacturing expenditure.
German Pat. No. 2,814,682 describes a press roll having a grooved, elastic cover, in which the grooves are widened at their bases. However, the ridges are thereby correspondingly weakened considerably at their "foot". There is a risk that the ridges will bend over under load so that the dewatering felt will thereby be damaged and/or the paper web will no longer be compressed with sufficient uniformity over its width.
A further attempt to solve the problem mentioned is described in International Application No. WO87/02080 with reference to an elastic press belt for a pressure shoe press. According to FIG. 2, the press belt comprises an elastomeric layer of material with a reinforcing fabric embedded in it. FIG. 3 suggests how the ridges are deformed in the press zone and how the grooves are narrowed. To avoid this, it is proposed that the two sides of the elastomeric layer of material be formed of different materials. In other words, the reinforcing fabric is to be coated with different types of plastic on each of its two sides. It is intended that the inside of the press belt which slides over the pressure shoe be less hard than the outside into which the grooves have been worked. However, production of this kind of belt is very expensive because the coating of the reinforcing fabric must take place in two separate operations and the press belt must be turned over in the interim. Furthermore, there is the danger that the two different elastomeric layers will come apart over time.
As is explained below, an attempt has already been made to increase the water holding capacity of the grooves in the press zone, despite the deformation of the ridges, by making the grooves wider. However, as a result, the grooves could be seen in the finished paper, i.e. so called groove marking is produced in an undesirable fashion in the paper (see U.S. Pat. No. 4,353,296, column 1, lines 32-37).